Scheme of two types of electron capture. top: The nucleus absorbs an electron. lower left: An outer electron replaces the "missing" electron. An x-ray, equal in energy to the difference between the two electron shells, is emitted. lower right: In the Auger effect, the energy released when the outer electron replaces the inner electron is transferred to an outer electron. The outer electron is ejected from the atom, leaving a positive ion.

Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron, thereby changing a nuclear proton to a neutron and simultaneously causing the emission of an electron neutrino. The nuclide, now in an excited state, then transitions to its ground state. An outer electron replaces the electron that was captured and an X-ray photon is emitted. Electron capture sometimes results in the Auger effect, where an electron is ejected from the atom and a positive ion results. Sometimes, a gamma ray is emitted because the nucleus is also temporarily in an excited state. Following electron capture, the nuclide's atomic number is reduced by one but there is no change in atomic mass.

If the energy difference between the parent atom and the daughter atom is less than 1.022 MeV, positron emission is forbidden as not enough decay energy is available to allow it, and thus electron capture is the sole decay mode. For example, rubidium-83 (37 protons, 46 neutrons) will decay to krypton-83 (36 protons, 47 neutrons) solely by electron capture (the energy difference, or decay energy, is about 0.9 MeV).

A free proton cannot normally be changed to a free neutron by this process; the proton and neutron must be part of a larger nucleus. In the process of electron capture, one of the orbital electrons, usually from the K or L electron shell (K-electron capture, also K-capture, or L-electron capture, L-capture), is captured by a proton in the nucleus, forming a neutron and emitting an electron neutrino.

Since a proton is changed to a neutron during electron capture, the number of neutrons in the nucleus increases by 1, the number of protons decreases by 1, and the atomic mass number remains unchanged. By changing the number of protons, electron capture transforms the nuclide into a new element. The atom, although still neutral in charge, now exists in an excited state with the inner shell missing an electron. An outer shell electron eventually makes a transition to replace the missing inner electron and thereby moves into a lower energy state. During this process, that electron will emit an X-ray photon (a type of electromagnetic radiation) and other electrons may also be emitted (see Auger electrons). Often the nucleus will be in an excited state also, and will emit a gamma ray as it transitions to the ground state energy of the new nuclide.

The electron that is captured is one of the atom's own electrons, and not a new, incoming electron, as might be suggested by the way the above reactions are written. Radioactive isotopes that decay by pure electron capture can be inhibited from radioactive decay if they are fully ionized ("stripped" is sometimes used to describe such ions). It is hypothesized that such elements, if formed by the r-process in exploding supernovae, are ejected fully ionized and so do not undergo radioactive decay as long as they do not encounter electrons in outer space. Anomalies in elemental distributions are thought[by whom?] to be partly a result of this effect on electron capture. Inverse decays can also be induced by full ionisation; for instance, 163Ho decays into 163Dy by electron capture; however, a fully ionised 163Dy decays into a bound state of 163Ho by the process of bound-state β− decay.[7]

Chemical bonds can also affect the rate of electron capture to a small degree (in general, less than 1%) depending on the proximity of electrons to the nucleus. For example in 7Be, a difference of 0.9% has been observed between half-lives in metallic and insulating environments.[8] This relatively large effect is due to the fact that beryllium is a small atom whose valence electrons are close to the nucleus.

Around the elements in the middle of the periodic table, isotopes that are lighter than stable isotopes of the same element tend to decay through electron capture, while isotopes heavier than the stable ones decay by electron emission. Electron capture happens most often in the heavier neutron-deficient elements where the mass change is smallest and positron emission isn't always possible. When the loss of mass in a nuclear reaction is greater than zero but less than 2m[0-1e-], the process cannot occur by positron emission but is spontaneous for electron capture.